This application is based on Patent Application No. 2001-103770 filed Apr. 2, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an ink jet printing apparatus which ejects ink from a print head to form an image. In particular, the present invention relates to an ink jet printing apparatus which has four or more color ink print heads arranged in a main scan direction and performs printing in both forward and backward scans. More specifically the present invention relates to a method of reducing color variations caused by changes in the ejection order of color inks.
2. Description of the Related Art
Printing apparatus generally applied to printers, copying machines and facsimiles print an image of dot pattern on a print medium, such as paper and a thin plastic sheet, according to image information.
Such printing apparatus can be classified into, for example, an ink jet printing system, a wire dot printing system, a thermal printing system and a laser beam printing system. An ink jet printing apparatus that uses the ink jet printing system projects ink droplets from nozzles of print heads onto a print medium to form an image on it.
As a variety of kinds of printing apparatus has come to be used in recent years, there are growing demands on these printing apparatus for higher printing speed, higher resolution, higher image quality and reduced noise. An example printing apparatus that can optimumly meet such requirements is the ink jet printing apparatus described above. Since the ink jet printing apparatus ejects ink from the print heads, the ink ejection operation and the amount of ink ejected need to be stabilized to meet the above requirements.
In realizing a further increase in the printing speed of the ink jet printing apparatus, it is considered essential to perform a forward-backward printing (or bi-directional printing) in which the printing is done in both the forward pass and the backward pass of the main scan of the print heads. When a color image is to be formed using the forward-backward printing, a problem arises that color variations are caused by changes in the printing order of inks.
A mechanism by which two color inks penetrate into a print medium will be explained by referring to FIG. 12. In print mediums (OHP and film-based medium) which absorb ink slowly, the dye/pigment particles of two color inks are mixed together as they soak into the medium, so that a hue difference caused by a change in the printing order of the color inks is relatively small. However, in print mediums (dedicated paper, glossy paper, etc.) that absorb ink quickly, since the dye/pigment particles of two inks penetrate and fix in the medium separately, the hue difference due to the change in the printing order is conspicuous.
In one embodiment of the present invention using six color heads arranged laterally side by side as shown in FIG. 14, color variations considered to be produced by a difference in the printing order between the forward pass and the backward pass are observed. When a G (green) image (not shown) is formed by printing in both the forward and backward passes, for example, the order of printing differs between the forward pass and the backward pass. That is, the C (cyan) is printed first followed by Y (yellow) in the forward pass thus producing a G image with a strong hue of cyan. In the backward pass, Y (yellow) is printed first followed by C (cyan) thus producing a G image with a strong hue of yellow. This alternate hue variation is recognized as bands at a pitch corresponding to the feeding distance of the print medium.
FIG. 11 shows an example of a multipass printing method that completes printing one print area with four print scans. A print head with 16 nozzles is divided into four equal nozzle groups, each of which prints through a thinning out mask pattern shown at the left end of the figure in all scans. The thinning out mask pattern can be set in the form of a fixed mask pattern or a random mask pattern. Pixels painted black represent those printed at each current print scan and pixels painted gray represent those already printed at or before the preceding scans. When printing is done with 25% thinning out, the image is formed with four print scans. How an image is formed using such a mask pattern is shown in FIG. 13.
Suppose a carriage M1002 is reciprocated to perform the bi-directional printing. When a plurality of heads ejecting different color inks are arranged side by side in the main scan direction as shown in FIG. 13, a color variation is observed which is considered to be produced by a difference in the printing order of the heads between the forward pass and the backward pass. This color variation appears in the form of bands at a pitch corresponding to the feeding distance of the print medium. In FIG. 14, HY represents a print head for ejecting a yellow ink; HM represents a print head for ejecting a magenta ink; HC represents a print head for ejecting a cyan ink; HML represents a print head for ejecting a light magenta ink; HCL represents a print head for ejecting a light cyan ink; and HK represents a print head for ejecting a black ink.
FIG. 11 shows a mechanism by which a color variation observed during a multipass printing is produced. In this example, four passes are performed to print one print area and the mask patterns used in the four scans are complementary to each other. When an image of a uniform secondary color of G (green) is to be formed by four print scans, as shown in the figure, the color print image are printed in the order from C to Y or from Y to C. In this case, the C ink which is printed first is adsorbed on the surface of the print media and the Y ink which is subsequently printed penetrates into the print medium in the direction of its depth. This phenomenon is considered due to the fact that because the surface portion of the print area to which the dye can attach is deprived by the first printed ink, the subsequently printed ink penetrates into the medium in the direction of its depth. Hence, the hue obtained with the C ink printed first and the hue obtained with the Y ink printed first differ, and this hue difference caused by the difference in the printing order of the two inks visibly appears as color variations, which degrade an image quality.
One example of a printed area formed by printing under the conditions of FIG. 11 is shown in FIG. 13. It is seen from the figure that there are density variations in the form of stripes or bands at a pitch corresponding to the paper feed distance.
As a measure to reduce the color variations, Japanese Patent Application Laid-open No. 6-336016 (1994), for example, discloses a technique of using an ejection mask pattern which comprises concentrated dot patterns elongate in the main scan direction as basic units. This technique has been verified to be effective for use with ink jet heads with a dot resolution of 300 dpi-600 dpi.
Further, Japanese Patent Application Laid-open No. 2000-37863 discloses a technique applied to ink jet print heads with a higher resolution of, for example, 720 dpi to 1200 dpi, in which each of the concentrated dot units is made relatively large, for example, 8 dots long and 16 dots wide. With this technique, even when an overlapping of dots occurs at a boundary between regions of different colors, it is possible to reduce color variations at an overlapping boundary portion caused by a change in the scan direction of the print heads.
However, as ink jet printers with higher resolutions of 720 dpi to 1200 dpi become available as a result of technological advance, the diameters of dots formed in Japanese Patent Application Laid-open No. 6-336016 (1994) also decreases further down to about 40 xcexcm to 50 xcexcm Even when the nozzle arrangement density increases, decreasing the amount of ink ejected from each nozzle and therefore the dot diameter, the landing error of ejected ink droplet does not change as much and thus becomes large relative to the dot diameter. As a result, it is becoming increasingly difficult to realize an intended high-resolution dot pattern on a print medium. Hence, in the forward-backward pass printing (bi-directional printing), simply applying the conventional design method of an ink ejection mask pattern to the high-resolution printing cannot effectively reduce the color variations and image disturbances that are likely to occur during the bi-directional printing.
Also in Japanese Patent Application Laid-open No. 2000-37863, since the unit size of the concentrated pixel group that produces a satisfactory effect of reducing the color variations is large in the high resolution printing method of recent years, a problem is observed in which a cyclic texture is easily visible on a printed image. Although this problem can be dealt with in an image forming that places an importance on sharp outlines, such as DTP and graphics, it is not possible to ensure a satisfactory quality with photographic images.
It is an object of the present invention to provide an ink jet printing apparatus in which a plurality of print heads are arranged in the main scan direction and which can reduce color variations and texture caused by a difference in the printing order between the forward pass and the backward pass.
To achieve this objective, the present invention provides an ink jet printing apparatus comprising: a plurality of print heads arranged in a main scan direction and having different inks, each of the print heads having a plurality of nozzle groups, each nozzle group having a plurality of ink ejection nozzles, the different nozzle groups in each of the print heads being scanned over the same print area on a print medium in forward and backward passes to complete an image on the print area by using a plurality of inks; and a print duty setting means for dividing a print duty setting area for each of the nozzle groups into a plurality of subdivided areas, for setting a print duty for each of the subdivided areas and for setting print duties of the print heads to different values.
In another aspect, the present invention provides an ink jet printing apparatus comprising: a plurality of print heads arranged in a main scan direction and having different inks, each of the print heads having a plurality of nozzle groups, each nozzle group having a plurality of ink ejection nozzles, the different nozzle groups in each of the print heads being scanned over the same print area on a print medium in forward and backward passes to complete an image on the print area by using a plurality of inks; and a print duty setting means for setting print duties of end portions of each of the print heads lower than print duties of other portions.
In still another aspect, the present invention provides an ink jet printing apparatus comprising: a plurality of print heads arranged in a main scan direction and having different inks, each of the print heads having a plurality of nozzle groups, each nozzle group having a plurality of ink ejection nozzles, the different nozzle groups in each of the print heads being scanned over the same print area on a print medium in forward and backward passes to complete an image on the print area by using a plurality of inks; and a print duty setting and modification means for switching a print duty distribution in a nozzle array direction between high and low values according to a frequency of use of the print head.
In further aspect, the present invention provides an ink jet printing method for an ink jet printing apparatus, wherein the ink jet printing apparatus includes a plurality of print heads arranged in a main scan direction and having different inks, each of the print heads having a plurality of nozzle groups, each nozzle group having a plurality of ink ejection nozzles, the ink jet printing method comprising the steps of: scanning the different nozzle groups in each of the print heads over the same print area on a print medium in forward and backward passes to complete an image on the print area by using a plurality of inks; dividing a print duty setting area for each of the nozzle groups into a plurality of subdivided areas; setting a print duty for each of the subdivided areas; and setting print duties of the print heads to different values.
In a further aspect, the present invention provides an ink jet printing method for an ink jet printing apparatus, wherein the ink jet printing apparatus includes a plurality of print heads arranged in a main scan direction and having different inks, each of the print heads having a plurality of nozzle groups, each nozzle group having a plurality of ink ejection nozzles, the ink jet printing method comprising the steps of: scanning the different nozzle groups in each of the print heads over the same print area on a print medium in forward and backward passes to complete an image on the print area by using a plurality of inks; and setting print duties of end portions of each of the print heads lower than print duties of other portions.
In further aspect, an ink jet printing method for an ink jet printing apparatus, wherein the ink jet printing apparatus includes a plurality of print heads arranged in a main scan direction and having different inks, each of the print heads having a plurality of nozzle groups, each nozzle group having a plurality of ink ejection nozzles, the ink jet printing method comprising the steps of: scanning the different nozzle groups in each of the print heads over the same print area on a print medium in forward and backward passes to complete an image on the print area by using a plurality of inks; and switching a print duty distribution in a nozzle array direction between high and low values according to a frequency of use of the print head.
As described above, since the print duties of the print heads arranged in the main scan direction are set to different values, the color variations caused by a difference in the printing order between the forward pass and the backward pass can be minimized and a high quality color image printed at high speed. Further, by switching the print duties for each print head between high and low values according to the state of use of the nozzles, it is possible to prevent partial degradation of the print head and thereby improve its service life.
Further, since the print duty setting area for each nozzle of each print head is divided into a plurality of subdivided areas and the print duties of the subdivided areas are set to different values, an image printed can be prevented from developing a texture and thus ensure a high image quality.
Further, since the print duties of nozzle groups situated at both ends of the print head are set low, it is possible to reduce the formation of blank lines caused by deviations of ink dots ejected from end portions of the print head as it moves during the printing operation. Therefore, a substantial improvement of image quality can be expected.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.